| Summary: | The stability of a deep composite roof is a powerful guarantee for the safe and efficient production of a coal mine. The coal–rock combination, the single rock or coal bodies have different bearing capacity; thus, we can accurately obtain the deformation field evolution and failure mechanisms of the combination, which is useful in the deformation control of a composite roof. In this study, based on the digital speckle correlation method (DSCM), a uniaxial compression test was applied to coal–rock combinations with different height ratios. The results revealed that the compressive strength, elastic modulus, and secant modulus of the combination gradually decreased, while the decreasing amplitude weakened with the increase of coal height. Additionally, the strain field map of the combination had different characteristics in different stages. As the height of the coal body continuously increased, the gradient of the strain cloud and the area of local strain increase moved upwards. Moreover, the cracks caused by the failure of the coal body in the combination triggered the failure of the rock body. According to the test results, two principles are proposed for the deformation control of the composite roof, and are expected to be useful in applications for the similar geological conditions.
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